Hydraulic control system for die pads in presses



Sept. 8, 1964 F. M. WILLIAMSON 3,147,723

HYDRAULIC CONTROL SYSTEM FOR DIE PADS IN PRESSES Filed May 24. 1961 2Sheets-Sheet 1 W 22 I Z2 M6 1 24 /44- y My IV A W/ A INVENTOR. 2 70 J MM7Z'k7/7J07Z irraFNI/S P 3, 1964 F. M. WILLIAMSON 3,147,723

v HYDRAULIC CONTROL SYSTEM FOR DIE PADS IN PRESSES Filed May 24, 1961 2Sheets-Sheet 2 i E 222 45 1' if l/ I i INVENTOR.

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United States Patent 3,147,723 HYDRAULIC CONTROL SYSTEM FOR DIE PADS KNPRESSES Floyd M. Williamson, Di-Dro Engineering Company, 2405 E. GrandBlvd, Detroit 11, Mich. Filed May 24, 1961, Ser. No. 112,439 9 Claims.(Cl. 11345) This invention relates to a press having hydraulicallyactuated die pads for applying pressure against the work during theworking stroke of the press and has particular reference to a hydrauliccontrol system for the die pads.

In a press of this type the movable die pad is provided with a hydrauliccushion consisting of a cylinder adapted to be supplied with hydraulicfluid under pressure from a surge tank and a piston reciprocating withinthe cylinder is urged by the hydraulic pressure fluid against the diepad to exert pressure on the die pad and to provide a predeterminedresistance to movement of the die pad as the press closes. The pressurefluid from the cylinder is exhausted during the working stroke of thepress back through a pressure relief valve which is set to open when apredetermined pressure is exceeded, thereby providing the desired degreeof resistance to movement of the die pad. In some press operations it isdesirable that the die pad exert different pressures against the workduring different portions of the working stroke of the press. Forexample, when deep drawing a metal part, relatively high pressure isneeded to grip the part at the beginning of the working stroke but, ifthis high pressure is maintained as the draw continues, the metal isliable to tear. It is therefore desirable to reduce the pressure exertedagainst the die pad as the working stroke progresses. Prior devices toaccomplish this result have not proven satisfactory for many reasons,principal among which is the fact that such prior arrangements undulycomplicate the hydraulic system and could not operate accurately in thefraction of a second allowed for their operation. The present inventionprovides a hydraulic control system which will automatically provide apredetermined degree of resistance to movement of the die pad during theinitial part of the working stroke of the press and then either increaseor decrease the pressure during the remaining portion of the workingstroke as may be required. The valve structure forming a part of thiscontrol system includes a timing valve in combination with a pair ofpressure relief valves. The pressure fluid in the hydraulic cushion isexhausted first through one of the pressure relief valves and then afterthe timing valve is actuated through the other of the pressure reliefvalves to provide different pressures on the die pad at differentportions of the working stroke of the press.

A principal object of the invention therefore is to provide a new andimproved hydraulic control system for die pads in presses.

Another object of the invention is to provide a hydraulic control systemfor a die padin a press which is adapted to apply a predeterminedpressure against the die pad during the initial portion of the workingstroke of the press and a difierent predetermined pressure against thedie pad during the remaining portion of the Working stroke of the press.

A further object of the invention is to provide a hydraulic controlsystem of the type described which includes a plurality of pressurerelief valves and a timing valve associated therewith for bringing thepressure relief valves into operation at predetermined times during theworking stroke of the press.

Other and further objects of the invention will be apparent from thefollowing description and claims and may be understood by reference tothe accompanying drawings, of which there are two sheets, which by wayof illustration show preferred embodiments of the invention and What Inow consider to be the best mode of applying the principles thereof.Other embodiments of the invention may be used without departing fromthe scope of the present invention as set forth in the appended claims.

In the drawings:

FIG. 1 is a sectional schematic view of a die pad for a press and ahydraulic control system therefor;

FIG. 2 is a sectional view through a die pad for use in performing adifferent type of operation in the press and which employs the hydrauliccontrol system shown in FIG. 1;

FIG. 3 is a sectional view showing a modified type of control system;

FIG. 4 is a sectional view taken on line 4-4 of FIG. 3;

FIG. 5 is a fragmentary sectional view illustrating a modified type ofdelay valve which may be used in the control system shown in FIG. 3; and

FIG. 6 is a sectional view of a further modified type of control system.

Referring to FIG. 1, there is illustrated therein a portion of a pressincluding a lower die shoe 10 having a forming die 12 carried therebyand an upper die shoe 14 provided with a punch 16 which is adapted toperform a drawing operation on the work 18 as the press is closed. Amovable die pad 20 is provided with a hydraulic cushion consisting ofcylinders 22 having pistons 24 therein which engage the die pad 20 toapply pressure against the work 18 as the press closes. The particulararrangement of the die members and the die pad 20 shown in FIG. 1 is forpurposes of illustration only and is merely intended to illustrate onetype of operation that may be performed in a press in which a relativelyhigh pressure is required to grip the work as the press closes and inwhich a relatively low pressure is required as the forming operationprogresses.

A tank 26 containing hydraulic fluid such as oil is supplied with airunder pressure through line 28 to maintain the oil in the tank 26 at apredetermined pressure. The oil is discharged from the tank 26 through aline 30 past a check valve 32 to a line 34 which intersects a conduit 36from which the oil under pressure is supplied to the cylinders 22 toforce the pistons 24 therein to their extended positions. As the presscloses the pistons 24 are retracted within their cylinders 22 and theoil in the cylinders is forced out back through conduit 36 to amultistage pressure relief valve indicated generally at 40 to provide apredetermined degree of resistance to movement of the pistons 24 duringthe working stroke of the press. The check valve 32 prevents a directreturn flow from the cylinders 22 back to the tank 26.

The valve 40 is provided with a cavity 42 containing a valve seat member44 at its upper end and a sleeve 46 engaging the valve seat member 44and communicating with the inlet passage 48 which communicates with theconduit 36. A cup-shaped valve member 50 reciprocates within the sleeve46 and is urged toward the lower end of the sleeve 46 by a spring 52 toa position in which it closes off radial ports 54 extending through thesleeve 46. The ports 54 communicate with a cavity 56 defined by areduced diameter portion of the sleeve 46. A passage 58 communicateswith the cavity 56 and with a line 60 which connects into the line 30leading from the tank 26.

The valve member 50 is provided with a small passage 62 to providecommunication between the inlet passage 48 and the interior of thesleeve 46. The valve seat member 44 is provided with an axial passage 64which communicates with the interior of the sleeve 46 and with aPatented Sept. 8, 1964.

valve chamber 66 containing a pressure relief valve 68 which engages aconical valve seat 70 formed on the valve seat member 44 to normallyclose the axial passage 64 in the valve seat member. The pressure reliefvalve 68 is urged to its closed position by a spring 72 which isconfined between the valve member 68 and a washer 74 which is engaged byan adjustable screw 76 by means of which the pressure of the spring '72may be adjusted thereby to regulate the pressure at which the pressurerelief valve 68 will open. A passage 78 leads from the valve chamber 66to the passage 58 and lines 60 and 30 back to the tank 26.

The valve body is provided with a cavity 80 containtaining a valve seatmember 82 and a sleeve 84 which defines a valve chamber 86 containing apressure relief valve member 88 which normally closes a passage 90 inthe valve seat member 82. The pressure relief valve member 88 is urgedto its closed position by a spring 92 confined between the valve member88 and a washer 94 which is engaged by a threaded adjusting screw 96 toregulate the pressure at which the valve member 88 will open. The valveseat members 44 and 82 are provided with radial passages 98 and 100respectively which are connected by a passage 102 in the valve body.

The valve is also provided with a cavity 104 containing a sleeve 106within which a time delay spool valve 108 reciprocates. A spring 110urges the spool valve 108 toward the right-hand end of the sleeve 106. Apassage 112 communicates with the chamber 86 through a radial port inthe sleeve 84 and with the interior of the sleeve 106 through a radialport therein. A passage 114 also communicates with the interior of thesleeve 106 through a radial port therein and the passages 112 and 114are normally closed off when the time delay valve 108 is in itsright-hand position as shown. A passage 116 provides communicationbetween the passage 100 in the valve seat member 82 and the right-handend of the time delay cylinder defined by the sleeve 106. A bleederpassage 116' leads from the left-hand end of the sleeve 106 andcommunicates with a passage 118 which intersects the passage 114 leadingback to the cavity 56 around the sleeve 46. An adjustable needle valve120 regulates the size of the passage 116.

When the press is opened the hydraulic cushion cylin ders are suppliedwith fluid under the pressure existing in the tank 26 through line 30,check valve 32 and lines 34 and 36. As the press closes, the hydraulicfluid is trapped in the cylinders 22 to resist movement of the pistons24 and the trapped fluid flows through line 36 and passage 48 in thevalve body 40 to the passage 62 in valve member 50. The oil flows fromthe interior of the sleeve 46 through passages 64, 98, 102, 100 and 116to react against the right-hand side of the timing valve 108. This valvecannot immediately shift to the left, however, because the fluid trappedin the left-hand end of the sleeve must escape past the needle valve 120which is adjusted to provide a desired time interval before the timingvalve 108 shifts far enough to the left to open passages 112 and 114.The valve member 68 is set to open at a higher pressure than the valvemember 88. However, since the passages 112 and 114 are initially blockedby the valve member 108, the pressure relief valve 88 will beinoperative at the beginning of the working stroke of the press and theexcess pressure in the cylinders 22 will at this time be relieved onlythrough the pressure relief valve 68. As soon as the pressure reliefvalve 68 opens, the pressure on the upper side of the valve member 50will be reduced, thereby enabling the pressure fluid to shift the valvemember 50 upwardly to open the radial ports 54 to return excess fluidthrough ports 54 and passage 58 to line 60.

The fluid may be bled off past the needle valve 120 at a selected rateso that at a predetedmined time during the working stroke of the presspassages 112 and 114 will be opened and communicate with each other pastthe reduced portion 122 of the valve member 108. As soon as this occurs,the valve member 88 will open and the consequent reduction in pressurein the system will cause valve member 68 to close, whereupon thepressure exerted on the die pad 20 will be determined by the setting ofthe pressure relief valve 88. The fluid for the remainder of the cycleof operation exhausts past the relief valve 88 to chamber 86, passage112 and the interior of sleeve 106 to passage 114, cavity 56 and passage58 to line 60. When the working stroke is completed, both pressurerelief valves 68 and 88 will be closed, valve member 108 will be shiftedto its right-hand position by spring and valve member 50 will againclose ports 54 under the influence of spring 52. The control system isthen set for another cycle of operation.

FIG. 2 illustrates a different type of forming operation on the work 18in which it would be desirable to cause the die pad to first exert highpressure against the work and then a lower pressure as the workingstroke is completed. An example of an operation of this type is atrimming and drawing operation in which the lower die shoe 10 isprovided with a trimming die and a forming die 142 while the upper dieshoe is provided with a die member 144 which is held in its extendedposition by a piston 146 reciprocating within a cylinder 148 whichcommunicates through line 150 with the conduit 36 shown in FIG. 1. Thepressure on the die pad is regulated by the pressure relief valvearrangement described in connection with FIG. 1 to exert a relativelyhigh pressure on the work during the trimming operation and then arelatively low pressure as the forming operation is carried out.

' In the embodiment of the invention shown in FIG. 3 the control systemis designed to provide high pressure during the initial portion of theWorking stroke of the press and a lower pressure at a predetermined timeduring the working stroke as in the previous modification. Theconstruction and operation of the valve are essentially the same in thismodification, although the time delay valve is of somewhat differentconstruction. In this modification the inlet passage 48 is in line withthe low pressure relief valve 88 rather than the high pressure valve 68.The parts comprising the pressure relief valves 68 and 88 and the valve50 may be identical to those employed in the FIG. 1 construction and aregiven the same reference characters. The fluid discharged from thecylinders 22 enters the passage 48 from the line 36 and flows throughthe passage 62 in valve member 50 and through passages 64 and 98 in thevalve seat member 44, passage 102 in the valve body and passages 100 and90 to the high pressure relief valve 68. When the pressure for which thevalve 68 is set is exceeded, the valve 68 will open to return the fluidto the tank through passages 78 and 58. The time delay valve includes avalve block 200' having a bore 202 containing a sleeve 204 within whicha time delay valve member 206- is slidably disposed. The valve member206 is urged by a spring 208 downwardly against a stop: plug 210 whichis engaged by a threaded adjusting screw 212 which may 'be regulated toadjust the length of the stroke of the of the relief valve 88communicates with a passage 220 in the valve block 200. The sleeve 204is provided with radial ports 222 which provide communication throughthe interior of the sleeve between the passage 220 and a passage 224.The upper end of the passage 224 communicates with the interior of thesleeve 204 above the valve member 206 through a passage 226 and a radialport 228 in the sleeve 204. The lower end of the passage 224communicates with a passage 230 which is connected to the passage 113leading to the cavity 56 within which the sleeve 46 is disposed.

When the press begins its working stroke, the valve member 206 blocksthe radial ports 222' in the sleeve 204 so that the pressure cannot berelieved through the relief valve 88. Thus high pressure relief valve 68Will open before valve 88. A passage 232 communicates with the passage116. in the valve body 40 and with a radial port 234 in the sleeve 204beneath the valve member 206. The pressure fluid supplied throughpassages 64, 98, 116, 232 and port 234 is effective to shift the valvemember 206 upwardly against the spring 208 to open the ports 222 as thereduced diameter portion 236 of the valve member is aligned with theports 222. The pressure fluid then opens the lowv pressure relief valve88 to reduce the pressure in the system and discharge the fluid throughpassages 112, 220,. 224, 230 and 114 tocavity 56 and passage 58 back tothe tank 26. When valve 88 opens, valve 68 closes because of thepressure drop.

The fluid trapped in the sleeve 204 above the valve member 206 escapesthrough port 228 to passages 226, 224, 230 and 114 to cavity 56 forreturn to the tank. The rate at which this trapped fluid escapes iscontrolled by an arcuate slot or groove 240 in the spool 214. The spool214 may be rotated to a desired position to vary the communication withport 228 thereby to regulate the time delay before opening of the lowpressure valve 88.

FIG. 5 illustrates a modified type of timing valve in which theconstruction is otherwise the same as that shown in FIG. 3. In this formof the invention the timing valve member 250 is constructed so that theports 222 in the sleeve 204 are open at the beginning of the workingstroke so that the low pressure relief valve 88 is effective at thebeginning of the working stroke. After a time interval determined by thesetting of the timing spool 2'14 andthe adjustment of the screw 212 thevalve member 250 will block the ports 222 to render the low pressurevalve 88. ineffective and the high pressure reliefvalve 68 willthereupon open to provide a higher pressure in. the cylinders 22 duringthe remainder of the working stroke of the press.

In the form of the invention shown in FIG. 6, the construction andoperation of the valve structure are essentially the same as that shownin FIG. 3 and the parts thereof are provided. with the same referencecharacters. However, the valve body 300 is of somewhat differentconstruction. In this modification the valve body is provided with apassage 302 which communicates with the interior of sleeve 204 and withthe inlet passage 48 so that the timing spool 206 is actuated bypressure fluid from passage 48. The valve member 206 initially blocksthe ports 222 in the sleeve 204 to delay the opening of the low pressurerelief valve 88 until after the opening of the high pressure valve 68 atthe beginning of the working stroke. As soon as the valve member 206 isshifted to the position shown, the ports 222 will be opened, whereuponthe pressure fluid will be discharged from the sleeve 46 throughpassages 64 and 98, passage 304 in the valve body 300, ports 222, aroundthe reduced portion of the valve member 206, passages 306, 100 and 90past the low pressure relief valve 88. The fluid is discharged from thelow pressure relief valve 88 through a passage 308 to the high pressurerelief valve chamber 66 and through passages 310 and 312 back to thesupply tank 26. The fluid trapped in the upper end of the sleeve 204escapes through passages 314 and 308 on the discharge side of the lowpressure relief valve 88 during the time that the latter is effective tocontrol the pressure. A passage 316, which is nornally closed by a checkvalve 318, communicates with a passage 320 communicating with a port 322in the sleeve 204 so that after the working stroke is completed thesleeve 206 may be rapidly filled on the return stroke of the press. Itwill be apparent that the valve member 206 could be modified to providea lower pressure in the system at the beginning of the Working stroke asin the modification shown in FIG. 5.

While I have illustrated and described preferred embodiments of myinvention, it is understood that these are capable of modification, andI therefore do not wish to be limited to the precise details set forthbut desire to avail myself of such changes and alterations as fallwithin the purview of the following claims.

I claim:

1. In a press having a movable die pad engaged by the work upon closingof the press and a hydraulic cushion for said die pad to move said padto its extended position and to provide a predetermined resistance tomovement of said pad to its retracted position upon closing of thepress, said hydraulic cushion comprising a cylinder having a pistontherein reacting against said pad, a source of fluid under pressure, afluid connection between said fluid pressure source and said cylinderthrough which pressure fluid is supplied to said cylinder to move saidpad to said extended position, a check valve in said connection toprevent return flow therethrough, a pair of return lines connecting saidcylinder to said fluid pressure source through which fluid is returnedto said source as said pad is retracted upon closing of the press, anormally closed pressure relief valve in each of said return lines, saidrelief valves being disposed in parallel relation to each other and setto open at diflerent predetermined pressures, timing means interposed inthe return line through the pressure relief valve which is set to openat the lower pressure, said timing means including a valve memberadapted in one position thereof to block said one return line and inanother position thereof .to open said return line, and means responsiveto the hydraulic pressure generated in said cylinder during closing ofthe press to shift said valve member from one of said positions to theother at a predetermined rate to effect discharge of fluid through eachpressure relief valve during. a predetermined portion of the workingstroke of the press.

2. In a press having a movable die pad engaged by the work upon closingof the press and a hydraulic cushion for said die pad to move said padto its extended position and to provide a predetermined resistance tomovement of said pad to its retracted position upon closing of thepress, said hydraulic cushion comprising a cylinder having a pistontherein reacting against said pad, a source of fluid under pressure, afluid connection between said fluid pressure source and said cylinderthrough which pressure fluid is supplied to said cylinder to move saidpad to said extended position, a check valve in said connection toprevent return flow therethrough, a pair of return lines connecting saidcylinder to said fluid pressure source through which fluid is returnedto said source as said pad is retracted upon closing of the press, anormally closed pressure relief valve in each of said return lines, saidrelief valves being disposed in parallel relation to each other and setto open at different predetermined pressures, and timing meansinterposed in the return line through the pressure relief valve which isset to open at the lower pressure, said timing means being operableresponsive to changes in the hydraulic pressure in said cylinder to openand close said one return line at predetermined times during each cycleof operation of the press thereby to eflect return flow past the higherpressure valve during a part of the Working stroke of the press andreturn flow through the lower pressure valve during the remainder ofsaid working stroke.

3. In a press having a movable die pad engaged by the work upon closingof the press and a hydraulic cushion for said die pad to move said padto its extended position and to provide a predetermined resistance tomovement of said pad to its retracted position upon closing of thepress, said hydraulic cushion comprising a cylinder having a pistontherein reacting against said pad, a source of fluid under pressure, afiuid connection between said fluid pressure source and said cylinderthrough which pressure fluid is supplied to said cylinder to move saidpad to said extended position, a check valve in said connection toprevent return flow therethrough, a pair of return'lines disposed inparallel relation to each other and connecting said cylinder to saidfluid pressure source through which fluid is returned to said source assaid pad is retracted upon closing of the press, a normally closedpressure relief valve in each of said return lines, said relief valvesbeing set to open at different pressures, and timing means operable inresponse to changes in the hydraulic pressure in said cylinder duringclosing of the press to etfect return flow of fluid through saidpressure relief valves in sequence thereby to change the pressure on thework at a predetermined time during closing of the press.

4. A press according to claim 3 wherein said timing means comprises avalve cylinder having a valve member therein adapted in one positionthereof to close the return line through the pressure relief valve whichopens at the lower pressure and adapted to open such return line inanother position thereof, a passage connecting said hydraulic cushioncylinder to said valve cylinder on one side of said valve member therebyto shift said valve member from one of said positions to the other uponclosing of the press, such movement of said valve member being resistedby fluid trapped in the other end of said valve cylinder, and meansproviding an adjustable bleed orifice through which the trapped fluidescapes from said valve cylinder to control the rate of movement of saidvalve member and thereby the time at which the pressure on the work ischanged.

5. A press according to claim 3 wherein said timing 6. In a press havinga movable die pad engaged by the work upon closing of the press and ahydraulic cushion for said die pad to move said pad to its extendedposition and to provide a predetermined resistance to movement of saidpad to its retracted position upon closing of the press, said hydrauliccushion comprising a cylinder having a piston therein reacting againstsaid pad, a source of fluid under pressure, a fluid connection betweensaid fluid pressure source and said cylinder through which pressurefluid is supplied to said cylinder to move said pad to said extendedposition, a pair of pressure relief valves through which fluid isreturned from said cylinder to said source as said pad is retracted uponclosing of the press, said relief valves being disposed inparallelrelation to each other and set to open at different predeterminedpressures, and means to delay the flow of fluid past the pressure reliefvalve which is setto open at the lower pressure so that fluid is firstreturned through the higher pressure relief valve during the firstportion of the working stroke of the press said means being operableafter the higher a pressure relief valve opens and in response to thehydraulic pressure generated in said cylinder during closing of thepress to permit fluid flow past said lower pressure valve during anotherportion of the working stroke of the press thereby to change thepressure on the work during said working stroke.

7. In a press having a movable die pad engaged by the work upon closingof the press and a hydraulic cushion for said die pad to move said padto its extended position and to provide a predetermined resistance tomovement of said pad to its retracted position upon closing of thepress, said hydraulic cushion comprising a cylinder having a pistontherein reacting against said pad, a source of fluid under pressure, afluid connection between said fluid pressure source and said cylinderthrough which pressure fluid is supplied to said cylinder to move saidpad to said extended position, a check valve in said connection toprevent return flow therethrough, a pair of return lines connecting saidcylinder to said fluid pressure source through which fluid is returnedto said source as said pad is retracted upon closing of the press, anormally closed pressure relief valve in each of said return lines, saidrelief valves being disposed in parallel relation to each other and setto'open at different predetermined pressures'in response to thehydraulic pressure generated in said cylinder during closing of thepress, and timing means for eflecting return flow through said pressurerelief valves in sequence and at predetermined times during the workingstroke of the press, said timing means comprising a cylinder having avalve member therein adapted in one position to close the lower pressurereturn line and in another position thereof to open such return line, afluid pressure transmitting passage connecting one end of said cylinderto said hydraulic cushion cylinder to shift said valve member from oneof said positions to the other, and a bleed passage leading from theother end of said valve cylinder to regulate the escape of fluid trappedtherein upon movement of said valve member thereby to elfect dischargeof fluid from said hydraulic cushion cylinder first through one of saidpressure relief valves and then through the other at a predeterminedtime during closing of the press.

8. A press according to claim 7 including means for regulating the sizeof said bleed passage to control the rate of movement of said valvemember.

9. A press according to claim '7 including means for varying the lengthof the stroke of said valve member.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A PRESS HAVING A MOVABLE DIE PAD ENGAGED BY THE WORK UPON CLOSINGOF THE PRESS AND A HYDRAULIC CUSHION FOR SAID DIE PAD TO MOVE SAID PADTO ITS EXTENDED POSITION AND TO PROVIDE A PREDETERMINED RESISTANCE TOMOVEMENT OF SAID PAD TO ITS RETRACTED POSITION UPON CLOSING OF THEPRESS, SAID HYDRAULIC CUSHION COMPRISING A CYLINDER HAVING A PISTONTHEREIN REACTING AGAINST SAID PAD, A SOURCE OF FLUID UNDER PRESSURE, AFLUID CONNECTION BETWEEN SAID FLUID PRESSURE SOURCE AND SAID CYLINDERTHROUGH WHICH PRESSURE FLUID IS SUPPLIED TO SAID CYLINDER TO MOVE SAIDPAD TO SAID EXTENDED POSITION, A CHECK VALVE IN SAID CONNECTION TOPREVENT RETURN FLOW THERETHROUGH, A PAIR OF RETURN LINES CONNECTING SAIDCYLINDER TO SAID FLUID PRESSURE SOURCE THROUGH WHICH FLUID IS RETURNEDTO SAID SOURCE AS SAID PAD IS RETRACTED UPON CLOSING OF THE PRESS, ANORMALLY CLOSED PRESSURE RELIEF VALVE IN EACH OF SAID RETURN LINES, SAIDRELIEF VALVES BEING DISPOSED IN PARALLEL RELATION TO EACH OTHER AND SETTO OPEN AT DIFFERENT PREDETERMINED PRESSURES, TIMING MEANS INTERPOSED INTHE RETURN LINE THROUGH THE PRESSURE RELIEF VALVE WHICH IS SET TO OPENAT THE LOWER PRESSURE, SAID TIMING MEANS INCLUDING A VALVE MEMBERADAPTED IN ONE POSITION THEREOF TO BLOCK SAID ONE RETURN LINE AND INANOTHER POSITION THEREOF TO OPEN SAID RETURN LINE, AND MEANS RESPONSIVETO THE HYDRAULIC PRESSURE GENERATED IN SAID CYLINDER DURING CLOSING OFTHE PRESS TO SHIFT SAID VALVE MEMBER FROM ONE OF SAID POSITIONS TO THEOTHER AT A PREDETERMINED RATE TO EFFECT DISCHARGE OF FLUID THROUGH EACHPRESSURE RELIEF VALVE DURING A PREDETERMINED PORTION OF THE WORKINGSTROKE OF THE PRESS.